1. Field of the Invention
The present invention relates to an optical module, and more specifically, it relates to an optical transmission/reception module ideal in applications in which light signals with at least two different wavelengths are exchanged through a single optical fiber.
2. Description of the Related Art
The single-fiber bidirectional communication achieved by using a single optical fiber through which two types of light signals with different wavelengths are propagated bidirectionally has become increasingly common in optical communication applications such as “fiber to the home” (FTTH). The bidirectional communication may be achieved by inputting light with two different wavelengths to a single optical fiber.
In such single fiber bidirectional communication, a light signal with a wavelength λ1 for transmission is originated at point A at one end of an optical fiber and a signal with a wavelength λ2 different from λ1 is originated at point B at the other end of the optical fiber. At point B, the light signal with the wavelength λ1 transmitted from point A is received and likewise, the light signal with the wavelength λ2 transmitted from point B is received at point A. Since the light signals with the wavelength λ1 and the wavelength λ2 are propagated along opposite directions through the optical fiber, a wavelength division multiplexer having a function of identifying and separating the individual wavelengths is normally installed at each end of the optical fiber.
FIG. 20 schematically shows a structure that may be adopted in single fiber bidirectional communication. In the example presented in FIG. 20, a wavelength division multiplexer 2a is connected to one end of an optical fiber 1 on point A side, and a laser diode (hereafter referred to as an LD) 3a and a photodiode (hereafter referred to as a PD) 4a are connected to the wavelength division multiplexer 2a. Likewise, a wavelength division multiplexer 2b is connected at an end of the optical fiber 1 on point B side, with an LD 3b and a PD 4b connected to the wavelength division multiplexer 2b. Light with a wavelength λ1 emitted from the LD 3a passes through the wavelength division multiplexer 2a, and is then identified at the wavelength division multiplexer 2b to be directed into the PD 4b. Likewise, light with a wavelength λ2 emitted from the LD 3b passes through the wavelength division multiplexer 2b, and is then identified at the wavelength division multiplexer 2a to be directed into the PD 4a. 
It is essential in further expansion of optical communication to make available a compact and affordable transmission/reception module with such a wave discriminating function and integrated functions for transmitting and receiving optical signals to facilitate widespread use at homes and offices.
In reference to FIG. 21, a transmission/reception module for single fiber bidirectional optical communication achieved in the related art is explained (refer to p208 “Receptacle Type Bidirectional Wavelength Multiplexing Optical Module I”, Masahiro Ogusu et al., Electronic Information Communication Convention, 1996, Electronics Society Seminar, C-208). In this module, a wavelength filter 21 achieving wavelength selectivity is fixed inside a rectangular parallelopiped housing 20, an optical fiber 27, an LD 22 and a PD 23 are fixed onto the external wall of the housing 20, and an optical fiber lens 24, an LD lens 25 and a PD lens 26 are fixed to the optical fiber 27, the LD 22 and the PD 23 respectively.
A light signal with a wavelength λ1 emitted from the LD 22 is converted to a parallel beam at the lens 25, is reflected at a 90° angle at the wavelength filter 21, is focused through the optical fiber lens 24 onto the optical fiber 27 and is propagated. A light signal with a wavelength λ2 having been propagated through the optical fiber 27 is converted to a parallel beam at the optical fiber lens 24, is transmitted through the wavelength filter 21 and is focused onto the PD 23 via the PD lens 26. This structure allows the module to function as a transmission/reception module for single fiber bidirectional optical communication.
However, the structure in the related art described above requires the optical fiber 27, the LD 22, the PD 23, the optical fiber lens 24, the LD lens 25 and the PD lens 26 to be positioned with exact accuracy for optical axis alignment, which poses a great deal of difficulty in reducing the manufacturing cost or achieving good mass productivity.